In nuclear reactors such as nuclear reactors cooled by water under pressure, it is necessary to vary the power supplied by the reactor as a function of the program of use of this reactor with which is associated a turbine enabling the driving of a turbo-alternator for the production of electric current.
Such variations in the power supplied by the reactor are obtained by variations in reactivity in the core of the reactor, i.e., in the zone in which the fuel assemblies are arranged.
In order to vary the reactivity of a pressurized-water nuclear reactor one employs firstly groups of control rods of material which absorbs the neutrons and which can be inserted between the fuel elements in the core of the reactor more or less completely, and secondly means enabling the concentration of soluble boron to be made to vary in the cooling liquid, i.e., the pressurized water.
In order to vary the concentration of soluble boron vary in the water under pressure, one introduces into this water either boric acid or pure water. The more or less strong concentration of boron in the water under pressure appears as a more or less high absorption of the neutrons by the boron in solution.
In pressurized-water nuclear reactors the fuel assemblies consist of bundles of tubular fuel elements containing the fuel material, each assembly forming an element of very great length with respect to its cross-section.
These assemblies are arranged side by side in the core of the reactor in a vertical position.
The control rods consist of tubular elements of the same length as the fuel elements and containing a material which absorbs the neutrons. These tubular elements are connected together so as to form a cluster which is movable as a whole in the vertical direction inside certain of the assemblies forming the core of the reactor. These clusters or control rods do not, however, move independently of one another; on the contrary they form groups within which the whole of the control rods move in the same way.
These groups of control rods being inserted into the core of the reactor from its upper part, a downward movement of a group consequently has a tendency to reduce the reactivity of the core of the reactor, whereas an upward movement of a group has a tendency to increase this reactivity.
A method of control of a pressurized-water nuclear reactor is known in which the groups of control rods are moved as much in one direction as in the other, according to a predetermined sequence. The movements of insertion or withdrawal of each of the groups of control rods employed in the reactor are therefore determined as a function of the movements of insertion or withdrawal of the other groups of control rods which precede them in a predetermined sequence.
In this method of control the control rods are moved only if a control parameter representative of a difference between the power demanded at the turbine and the real power from the core lies outside a predetermined range, called the deadband, straddling the value 0. On the other hand, the speed and the direction of movement of the groups of control rods are determined solely as a function of the value and of the sign of the control parameter.
This control parameter of the reactor is generally homogeneous at one temperature and is calculated by difference between the temperature of the core and a reference temperature which is a function only of the power demanded at the turbine.
In this way the power supplied by the reactor, which is a function of the temperature of the core, may be adapted to the power demanded at the turbine.
However, variations in power being obtained by a variable insertion of the control rods along the height of the core of the reactor, the distribution of the neutron flux along the height of the core, or the axial distribution, becomes disturbed. The same applies to the axial distribution of power which is similar to the distribution of neutron flux.
Hence the control of the nuclear reactor by insertion of control rods may cause the appearance of axial distributions of power which are extremely remote from an ideal distribution which is that, i.e., observed in the core when the control rods are inserted only a little way into the core.
Hence this distribution of power may develop towards a very unfavorable state which causes the appearance of hot points in the core of the reactor which may even terminate in local destruction of the fuel elements.
In order to avoid this unfavorable development of the axial distribution of power in the reactor core, it is therefore necessary to have recourse to a means of control of the reactivity in the core which is different from the control rods. Hence, when the axial distribution of power develops towards an undesirable distribution, a variation is caused in the concentration of soluble boron in the water under pressure until the time at which the level of power desired may be obtained solely by the action of the soluble boron. The control rods are then returned into a position enabling a satisfactory axial distribution of power to be obtained.
On the other hand, the disturbances of the axial distribution of power are again aggravated at the time of variations in power of the reactor by phenomena such as the formation or the disappearance of xenon by nuclear reaction. In order to compensate these effects, which further increase the unbalance in the distribution of power, one likewise possesses as the means of action only soluble boron.
This presents disadvantages because the action of the soluble boron is not instantaneous and the putting to work of this means of regulation must be effected manually.
Hence this method is difficult to employ in the case where it is required to modify the power of the reactor rapidly.
In addition, a circuit must be provided which enables either the introduction of boric acid or the introduction of water rapidly and in perfectly proportioned amounts into the reactor cooling fluid.